The significance of coherent flow structures for the turbulent mixing ...

6.1 Experimental set-up
the (virtual) slave cameras 1 and 2, located behind the mirrors, are not presented in the table
as these can be considered as identical with the corresponding master cameras 3 and 4.
y
x
main flow-direction
c
a
10 wall-units light-sheet separation
~94°
b
8
7
6
5
4
2
1
3
FIGURE 6.2: Experimental set-up for ÚQ -investigation. 1-4 digital cameras, 5 lens, 6 mirror, 7 polarising
beam-splitter cube, 8 absorbing material.
camera á [mm] Ä [mm] OP [mm] R [deg]
3 628 -573 850 47.1
4 -608 -565 829 47.6
TABLE 6.1: Camera position, observation
distances and viewing angles with respect
to the centre of the field of view.
For magnification and field of view adjustments each Scheimpflug-adapter was mounted
on a two-axis micrometer translation stage, and the polarising beam splitter-cubes and mirrors
in front of the lenses were connected to two-axis tilt-rotation stages and gimbal mirror mounts.
To obtain ideal particle images for the image analysis algorithms ÓöÎ
(bright circles, pixel in
diameter, surrounded by a dark background), the imaging of the field of view was performed
by means of 100 mm Carl Zeiss lenses with an aperture of 8. This leads to a complete erasure
of all optical aberrations and out-of-focus effects. The arrangement was installed
below
the wind-tunnel, as shown in figure 6.2. The mean observation S
distance was mm and
*:U
the
opening angle between the left and right camera systems was T set to to resolve the out-ofplane
motion with sufficient accuracy according to figure 3.5. This is important because the
out-of-plane component is required to calculate the dominant Reynolds shear-stress component
V turb W
glued on an aluminium plate and attached with a micrometer translation stage in such a way
that a parallel motion of the grid could be achieved in vertical direction. This grid was aligned
with each light-sheet one after another and recorded each time with the four cameras (before
and after the experiment in order to proof the conservation of the boundary conditions during
the experiment). As any horizontal translation of the target could be excluded with this device,
Î& 6(7 . For the calibration of the system a regular grid with Ó mm line spacing was
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